Laser Optics

The power density required to cut sheet metal.

Sheet metal cutting requires a power density of 10^6 watts/in^2 (source: Mike Klos @ laser mechanisms)

Converting to millimeters, that's 1550 watts/mm^2. (using equation: 1in^2 = 645mm^2)

A 100 watt laser can achieve a power density of 1550 watts/mm^2 in a spot size that is 0.6452mm^2

A spot size that is 0.6452mm^2 has a diameter of of .28mm or 280 micron (using area = pi * (d/2)^2)

280 micron! If I can deliver 100 watts to a spot of 280 micron, I should be able to cut metal. That's too easy.

Why? Well, how big a diameter can I expect with my optics? The information on my beam diameter varies. I have read it goes anywhere from 1.6 to 2.3 mm.

At 1.6mm, if I have a 3x beam expander I get 4.8 mm, which will be 103 micron using a 1.5 inch focal length

(equation: diameter = .013 * M^2 * (fl/D) where M^2 is equal to 1, and D is diameter of incoming beam. See this site

If I substitute in an M^2 of 1.5, I still get a diameter of 150 micron. So according to calculations I should be able to deliver a power density needed is 10^6 watts per square inch.

Note: I'd like hear from anyone who could verify that 10^6 watts per inch is the power density I need.

Note: the reason I bought the microscope was to be able to measure in micron -- hopefully I can use it to check my beam diameter

Romos gave me some excellent feedback on my post about beam sizes. He points out that the expected beam size can be taken from this table for the G100:

Distance From Laser (mm) vs Beam Diameter (mm)

0 mm distance = 1.9 mm beam diameter
250 mm distance = 2.9 mm beam diameter
500 mm distance = 4.7 mm beam diameter
750 mm distance = 6.7 mm beam diameter
1000 mm distance = 8.7 mm beam diameter
1500 mm distance = 12.9 mm beam diameter
2000 mm distance = 17.2 mm beam diameter

In his case, the focal lens from the laser dinstace is 500 mm. So, without any beam expander I have...(assume that M^2 = 1.5)

diameter = .013 * 1.5 * (38.1/4.7) = 0.158mm

The distance to my beam expander is 33cm, so using that chart the beam size will be about 3.5mm when it goes into the expander. The beam expander is 3 times the original size so the beam will go to 10.5mm.

Based on the equation:

diameter = .013 * 1.5 * (38.1/10.5) = 0.071mm

This is a great spot size. The problem will be my depth of field. This is based on the formulas shown on this site:

http://www.parallax-tech.com/faq.htm

Depth of field is the distance range that an object can be placed in front of the lens and still get cut. The forumula for depth of field is

DOF = 2.5 x wavelength x ( focal_length / beam_diameter )^2

for the G100 laser it calculates to:

DOF = 0.027 * (focal_length / beam_diameter)^2

I made a chart that gives examples of our these two situations.

	Focal len(in)	Focal len(mm)	Beam Dia(mm)	Spot (micron)	DOF (mm)
Romos	1.5	38.1	4.7	158	1.8
Current	1.5	38.1	10.5	71	0.4
No expansion	1.5	38.1	3.5	212	3.2
Longer FL	4	101.6	10.5	189	2.5

The first row is what happens with a set up for another G-100 owner named Romos. He has a focal length of 38 mm, beam diameter of 4.7, giving a 158 micron spot size, and 1.8 mm depth of field. My current situation I have a 10.5 beam diamter, a 71 micron spot, and a small small depth of field of less than a milimeter. That is may work, but it was also useful to look at some other examples.

I tried to other variations. One would be if I had no beam expansion. My beam diameter would be around 3.5 (close to yours) and a slightly larger spot size. This might work. If you look at the first post it said I have to get at least 280 micron spot size, so it could be enough power density to cut metal.

The last row of the chart is where I use a focal length of 4 inches with the beam expander. If I do that, I get a spot size of 190 micron, and 2.5mm depth of field.

It is useful to have more more than one focusing lens anyway, because I will be cutting other materials such as wood where a longer focus length would be helpful.

The optics for the laser

The principal of the cutting head is that the beam enters the top of the head and is directed to a focusing lens that is found in the center of the cutting head cavity. A focused beam exits through the bottom of the cutting head nozzle. Gas, such as oxygen, is fed into the side of the chamber below the focusing lens. This gas exits the nozzle along with the beam and the laser beam/oxygen combination serves to vaporize the steel for cutting.

See this video of the laser head in action.

Alignment

The place your targets on the beam path. If the item that gets the tape can be threaded into place it makes it easy to mount the target.

Using this system, I started with a target on the cut quality enhancer, and then moved on to the elbow that points the beam towards the floor. The elbow has allen head screws that allow you to microadjust the mirrors in the beam. This took a little while to figure out the impact of changing these screws and where the beam lands, so for a while I would take to shots on one piece of paper, and view the where the beam moved after making a change. After I got the hang of this, I went back to the targetting system to adjust the beam as best I could to be on center.

The cutting head has a nozzle on it with a port that is roughly half a millimeter in diameter. If the beam is not exactly on center, it gets reflected off the side when it comes out of the nozzle and forms a characteristic pattern that looks like this:

(picture courtesy of Romos)

Another alignment method I used to cure this problem was to remove the nozzle, and shine a short pulse on thermally sensitive paper. There are some examples of that this looks like below. Carefully adjust the beam so that it produces the same spot shape with and without the nozzle to ensure it is going directly through the port of the cutting head. Romos also recommended that acrylic works as an alternative to the thermal paper.

Height adjustment.

To find the best height for minimum beam diameter, I used the thermally sensitive paper and looked at the beam diameter as a function of height. The markings on the card are based on 100ths of an inch are relative; they do not reflect the actual distance of the focussing lens to the paper. What you can see from this experiment is that the beam size gets smaller down to a distance of 650ths of an inch and then starts to increase in size.

I would not claim that is a good method to determine the beam diameter. I dont know if there is a way to determine what the beam size is, however, it was still interesting to look at the spot under the 100x microscope. This is a picture of my smallest possible spot on the thermal paper. The microscope was focussed on the stainless steel below the paper. You can see burnt edges around the hole. The burns are not a result of reflection as shown here, at least they dont occur like this repeatedly. It seems more like the results of a heat flare coming off of the beam.

This is a picture of the underside of the paper on the 100x microscope. The scope has a grid inside of its optics. Large lines are 100 micron apart. From this picture you can see the hole is roughly 200 micron.

This is the underside of 1/8th inch thick plywood which was cut at 10% power levels on the laser. The kerf width is roughly 200 micron as well.

A video of a short pulse for the laser. I dont know the actual time length of the pulse. Its based on a microcontroller program that creates pulse lengths that increase by a factor of ten. By flipping between different lengthed pulses I was always able to find a pulse that worked well enough for my application. This is a longer length video.